1. Field of the Invention
The present invention relates to a communication system, and an apparatus therefor, for communication of audio or image data, and more particularly to a communication system, and an apparatus therefor, adapted for connection to a digital communication network such as integrated service digital network (ISDN)
2. Related Background Art
One communication apparatus of this kind is known, as disclosed in the U.S. Pat. No. 4,353,097, having a telephone function and a facsimile function and capable of switching from said facsimile function to said telephone function without interrupting the connection between the transmitting and receiving units
However, in such apparatus, the switching of function is tedious in practice, because, after the selection of switching by the operator, the function has to be switched from the facsimile to the telephone by the operators of the transmitting and receiving units.
Also in case of facsimile communication after telephone communication in such conventional apparatus, the facsimile communication can only be started with a cumbersome procedure, after the switching from the telephone function to the facsimile function, by the actuation of a transmission key by the operator at the transmitting side and the actuation of a reception key by the operator at the receiving side. Besides, the operator can operate the transmission key or the reception key without moving if the telephone function and the facsimile function are incorporated in a single unit, but, if the telephone unit is positioned distant from the facsimile unit, the operator has to move from the location of said telephone unit to that of said facsimile unit in order to operate the key. Such operation is not only cumbersome but also deteriorates the efficiency of communication.
Particularly in an integrated service digital network (ISDN), the user units are often positioned mutually distant, because eight telephone units and data communication units at m can be connected, through a bus-like line in each subscriber's home, to a network terminal (NT) which is in turn connected to a subscriber line. Consequently, in case the connection is switched from a telephone unit to a facsimile unit without interrupting the connection by so-called change-of-service function, known as one of the services available in such ISDN, the operator has to move for operating the transmission key as explained above. Such operation is cumbersome and deteriorates the efficiency of communication.